The invention relates to methods and systems for detecting and removing self-alerts in a tracking system and, more specifically, methods and systems for detecting and removing self alerts, i.e., targets representing one""s own aircraft, in an air-to-ground tracking system.
TIS (Traffic Information Service) is a technology in which air traffic control Secondary Surveillance Radar (SSR) on the ground transmits traffic information about nearby aircraft to any suitably equipped aircraft within the SSR range. The transmissions are addressed to a particular aircraft, and are sent together with altitude or identity interrogations. In turn, the aircraft to which the transmissions are addressed receives such and displays to the pilot information about nearby aircraft that are being interrogated by the SSR.
Due to numerous reasons (e.g., signal reflection, fly in-out TIS coverage area, deflected Mode Stamp orders) the ground station may misinterpret the responses from interrogations. As a result, the ground station may misinterpret the addressed aircraft as another aircraft and uplink a false alert (i.e., a self-alert). Thus, it is desirable in TIS systems to be able to efficiently distinguish uplinked information identifying one""s own aircraft (a TIS self-alert) from information identifying a distinct target that may pose a threat of mid-air collision. TIS self-alerts are undesirable because they can distract a pilot""s attention from real threats or disturb essential flight operation (e.g., taking off or landing).
As is known by those of ordinary skill in the art, each TIS message or broadcast that is sent from the ground radar station will typically include the following information for each nearby aircraft (target):
1. Traffic Bearing defined as the angle from the ownship to the target aircraft with respect to the ownship track over the ground, quantized in about 6-degree increments.
2. Range defined as the distance between the ownship and the target aircraft, quantized in about 0.125-nm (nautical mile) increments.
3. Relative Altitude defined as the difference in altitude between the target aircraft and the ownship, quantized in about 100-foot increments. A positive value indicates that the aircraft is above ownship, while a negative value indicates that the aircraft is below the ownship.
4. Altitude Rate is an indicator of whether the target aircraft is climbing (value of 1) or descending (value of 2) faster than 500 feet per minute or in level flight (value 3).
5. Track is defined as the ground track angle of the target aircraft quantized to 45-degree increments.
Based empirically on flight test data, TIS self-alerts can generally be defined as those TIS targets having; (a) range less than 0.125 (xe2x85x9) nm from ownship and (b) relative altitude of less than 200 feet. Thus, a simplified approach would be to identify as a self-alert any TIS target that satisfies the above conditions (i.e., the self-alert conditions) and properly filter out the TIS self-alert so that it would not appear on the CDTI (Cockpit Display of Traffic Information) in the aircraft""s cockpit. However, this approach would be unacceptable because it would not adequately address for example; (1) a TIS self alert that occasionally falls outside of the self-alert conditions, or (2) a real TIS target disappearing from the CDTI when it gradually flies into the self-alert conditions. Hence, while enlarging the self-alert conditions would solve (1) it would, conversely, heighten the problems related to (2).
A basic assumption can be made that a TIS self-alert must have its track very close to the ownship track since, by definition, a TIS self-alert is a reflection of ownship. Using this assumption, it would be possible to construct a TIS self-alert filtering scheme that provides for enlarged self-alert conditions (e.g., (a) range less than 0.875 nm from ownship and (b) relative altitude of less than 500 feet) and has TIS self-alert track close to the ownship track. This method of TIS detection will be referred to herein as a simple filtering algorithm.
However, this type of TIS self-alert filtering scheme would create an additional drawback; (3) a TIS target that flies in formation with the ownship will be misidentified as a self-alert and hence, will be filtered out. To rectify this situation, the self-alert conditions are further extended (e.g., (a) range less than 1.625 nm from ownship and (b) relative altitude of less than 1000 feet). In addition, when a TIS target is within these extended self-alert conditions, the TIS target will be preserved (i.e., not deemed a self-alert), if at least one TIS target has appeared in the extended conditions in the previous TIS upload message. This type of extended condition filtering also addresses drawback (2) related to a real TIS target disappearing from the CDTI when it gradually flies into the self-alert conditions.
However, even in the extended condition filtering scheme scenario, various problems present themselves that lead to TIS self-alert inaccuracies. For instance, in certain states the ownship track will jump significantly between consecutive TIS upload messages. Additionally, in certain states the tracks of the TIS self-alert will jump significantly between consecutive TIS upload messages. The simple filtering schemes and the extended condition filtering schemes discussed above do not take into account these significant changes in tracks between consecutive TIS messages. This drawback affects the ability of the filtering scheme to associate a TIS self-alert with ownship.
Another example of the inaccuracies presented by the previously discussed filtering schemes is that in certain states the altitude of the TIS self-alert will jump significantly (about 1000 feet) or becomes unknown between consecutive TIS upload messages. In this situation the above-discussed filtering schemes would be disabled from checking if a TIS target is within the self-alert conditions. Once the filtering scheme is disabled, a TIS self-alert would appear on the CDTI and, in turn, disablement would cause all other self-alerts included in the next consecutive upload message to appear on the CDTI. In a similar regard, a TIS self-alert will be undetected if one real TIS target were to appear in the extended conditions of the previous TIS message.
Thus, an unsatisfied need exists for an efficient and reliable method for detecting and removing TIS self-alerts. The desired detection and removal method and system should provide advanced TIS target correlation between two consecutive TIS upload messages. In addition, improved systems and methods for detecting and removing TIS self-alerts should provide a more flexible mechanism to minimize the impacts of TIS unstableness.
The present invention is an improvement in detecting TIS self-alerts from TIS upload messages to thereby eliminate the likelihood of TIS self-alerts appearing on CDTI displays. The present invention determines whether a self-alert was present in a previous traffic information uplink to determine whether a target in a current information uplink is real. Using a weighting algorithm, typically a fuzzy logic algorithm, a system in accordance with the present invention can distinguish between targets in consecutive radar sweeps to identify self-alerts. If the self-alert was not present in previous uplinks, then the system removes the self-alert from subsequent views on the CDTI. Thus, the present invention provides an improved visual display system for pilots and air traffic controllers. The invention also provides a more accurate visual display for pilots and air traffic controllers tracking actual targets in the vicinity of an airborne aircraft.
In accordance with an aspect of the invention, a weighted sum of the changes between traffic range, traffic track, relative altitude, and altitude rate in consecutive TIS messages for a target that may be a self-alert is determined. It has been determined empirically, through flight-test data, that the changes between consecutive range and track reports of a real TIS target are more stable than the relative altitude and altitude rate reports for the same real target. Thus, the weight applied to the delta range and delta track is higher than the weight applied to the delta relative altitude and delta altitude rate. If the weighted sum is less than a threshold, then two TIS targets from consecutive uploaded messages are determined to be associated (i.e. an actual legitimate target). If a TIS target appears within the pre-defined TIS self-alert cylinder and is not associated with any TIS target in a previous message, the TIS target is identified as a self-alert and is filtered so that it does appear on the CDTI.
In a preferred embodiment, the system according to the invention applies particular weights based on empirical flight-testing. The delta range and delta track values are weighted by the factor 0.1, while the delta relative altitude and delta altitude rate are weighted by the factor 0.07, and the threshold is set to 0.74. If the total weighted sum is less than the threshold, then the two TIS targets from consecutive uplinked messages are associated. If a TIS target appears within the pre-defined TIS self-alert conditions and is not associated with any previous TIS target from the other uplinked message, then the TIS target is identified as a self-alert and is filtered out.